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a Dep. of Natural Resources and Environ. Sci
b Dep. of Crop Sci., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801
c Growmark, 1701 Towanda Ave., Bloomington, IL 61701
* Corresponding author (mulvaney{at}uiuc.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: LSD, least significant difference • PPNT, preplant NO3 test • PSNT, presidedress NO3 test
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Public concern that excessive N fertilization may contribute to NO3 enrichment of ground and surface water has stimulated interest in soil testing to improve the accuracy of N fertilizer recommendations for corn. This concern may well be justified, since crop responsiveness to N fertilization can vary widely even within the same field (Harrington et al., 1997), and nonresponsive sites have been detected throughout the north-central and northeastern USA (e.g., Bundy and Malone, 1988; Fox et al., 1989; Roth and Fox, 1990; Meisinger et al., 1992; Brown et al., 1993; Schmitt and Randall, 1994). For many years, a preplant NO3 test (PPNT) has been used in western Canada and the Great Plains region of the USA to account for carryover of mineral N from previous cropping (Dahnke and Johnson, 1990; Bundy and Meisinger, 1994). Though originally developed for use in semihumid areas where leaching is limited, the PPNT has recently been applied in humid regions of the north-central USA to detect residual NO3 in the surface 60 cm of medium- to fine-textured soils (Bundy and Malone, 1988; Bundy et al., 1992; Schmitt and Randall, 1994). To improve the reliability of NO3 testing as a basis for fertilizer recommendations in humid regions, a presidedress NO3 test (PSNT) was developed by Magdoff et al. (1984), in which soil sampling is postponed until corn is 15 to 30 cm tall (V6 growth stage), so as to estimate plant-available NO3 as closely as possible to peak uptake by the crop. If the test indicates a low concentration of soil NO3 in the surface 30 cm (<20–30 mg N kg-1), supplemental N is applied as a sidedressing.
An inherent limitation with soil testing for NO3 arises from the dynamic nature of N-cycle processes, most of which affect soil NO3 concentrations. Ideally, a soil test for N would estimate the supply of organic N that produces NO3, but this approach can only be successful if a specific soil N fraction is identifiable that mineralizes readily and is directly related to fertilizer-N responsiveness. Several studies have been reported to compare the distribution of organic N in different soils, or among soils under different management practices (e.g., Stevenson, 1957; Keeney and Bremner, 1964; Porter et al., 1964; Moore and Russell, 1968; Sowden, 1968; Khan, 1971; Smith and Young, 1975; Meints and Peterson, 1977; Osborne, 1977). The results have generally indicated little variation in the distribution of hydrolyzable soil N, regardless of soil type, cropping, or cultivation. Such uniformity can be attributed, at least in part, to serious defects in methodology, since recent work in our laboratory has shown that conventional steam-distillation techniques for estimating amino sugar N and amino acid N are not quantitative (Mulvaney and Khan, 2001). The latter problem was eliminated by developing simple diffusion methods for N-distribution analysis of soil hydrolysates that are accurate, specific, and reliable.
The present project was motivated by the lack of fertilizer-N response observed at many of the sites used by Brown et al. (1993) to compare different N recommendation systems for corn, including the proven yield approach described in the Illinois Agronomy Handbook (1998) and soil NO3 tests before (PPNT) or after (PSNT) planting. Excessive accumulations of NO3 were not detected for many of the nonresponding sites, and in such cases overfertilization occurred with all three systems. The primary purpose of the work reported here was to test the hypothesis that nonresponsive sites are detectable on the basis of a specific soil N fraction, by comparing N-distribution analyses for responsive and nonresponsive soils using the diffusion methods described by Mulvaney and Khan (2001). To confirm the identity of this fraction, a comparison also was made of potential mineralization by incubating responsive and nonresponsive soils, while monitoring changes in hydrolyzable forms of soil N.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Soil Samples
Soil samples were collected from the experimental area at each site in late March or early April, including surface (0–18 cm) samples for routine soil fertility assessment (pH, P, and K) and profile (0–30 and 30–60 cm) samples for the PPNT. Additional sampling was done to a depth of 30 cm for the PSNT when corn was 15 to 30 cm tall in late May or early June. Surface samples were obtained as a composite of five 2.5-cm-diam. cores, which were air-dried at room temperature, crushed to pass a 2-mm screen, and then mixed thoroughly prior to analyses for pH, P, and K. Soil pH was measured with a glass electrode (soil/water ratio, 1:1), available P was estimated by the Bray-1 procedure (Bray and Kurtz, 1945), and exchangeable K was determined by flame emission spectroscopy following extraction with 1 M NH4C2H3O2 (Warncke and Brown, 1998).
For NO3 testing (PPNT and PSNT), five soil cores were collected from each block, combined, and subsequently frozen (-10°C) within 12 h after collection. Before analysis, the frozen cores were allowed to thaw at room temperature, screened to <2 mm while still field-moist, and a 10-g sample was extracted with 100 mL of 2 M KCl. The extracts were analyzed for NH4–N and (NO3 + NO2)-N by steam distillation techniques using MgO with or without Devarda's alloy (Mulvaney, 1996). The latter analyses were assumed to represent NO3–N in the surface 30 cm (PSNT) or 60 cm (PPNT, obtained as the mean of data for 0–30 and 30–60 cm samples) of the soil profile, and a correction was made for soil moisture content so as to express the results on an oven-dry basis. The remaining soil (<2 mm) was allowed to air-dry at room temperature, and was then transferred to polyethylene or paper bags for storage.
The soils used in the present project (Table 1) were PPNT samples (0–30 cm) from 18 of the 75 sites studied by Brown et al. (1993). The particular samples used, representing a wide range of textural classes and management practices, were selected from sites receiving normal rainfall, and included 11 sites that were nonresponsive and seven sites that were responsive to N fertilization. In each case, a composite sample was prepared by combining equal weights of the replicate samples, so as to integrate block effects in comparing different sites. The entire sample was ball-milled to pass through a 0.15-mm screen and was then stored in a mason jar sealed with an air-tight lid.
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Soil Hydrolysates and Hydrolyzable Nitrogen
To prepare soil hydrolysates, 5-g samples of soil (four replicates) were heated (110–120°C) under reflux for 12 h in 125-mL Erlenmeyer flasks fitted with a 24/40 ground-glass joint for attachment to a 40-cm Liebig condenser, after treatment with 20 mL of 6 M HCl and two drops of octyl alcohol. The hydrolysis mixture was filtered through Whatman no. 50 filter paper (Whatman, Clifton, NJ) in a 5.5-cm-diam. polypropylene Büchner funnel under vacuum, during which rinsing was done two to three times with 10 mL of deionized water from a wash bottle to complete transfer of the hydrolysate from the flask to the funnel, and the same rinsing process was repeated to ensure removal of the hydrolysate from the soil. The filtrate was collected in a 125-mL polyethylene bottle fitted with a screw-cap lid, and was stored in a refrigerator (5°C). Prior to use, the hydrolysate was transferred to a 250-mL beaker, and 10 M NaOH was added while monitoring pH with continuous stirring, so as to obtain a pH of approximately 4.0. Neutralization was continued in the same manner using 1 M NaOH, until a pH of 6.5 to 6.8 had been achieved. The neutralized hydrolysate was diluted to 100 mL with deionized water, and then was returned to the bottle originally used for storage and kept under refrigeration.
Using the diffusion methods described by Mulvaney and Khan (2001), neutralized soil hydrolysates were analyzed for total hydrolyzable N, NH4–N, (NH4 + amino sugar)-N, and amino acid N. Amino sugar N was determined as the difference between NH4–N and (NH4 + amino sugar)-N.
Laboratory Incubation Experiment
Five of the 18 soils were selected for use in an incubation study to evaluate potential mineralization and detect changes in hydrolyzable N fractions. The particular soils used included the nonresponsive sample that had received the highest manure application and had the highest content of organic C, total N, and hydrolyzable N; two additional nonresponsive soils which had received little, if any, manure; and two responsive soils, including the one that was highest in total and total hydrolyzable N and another that had the lowest content of C and N and was the most responsive to N fertilization.
To carry out incubations, 5-g samples of air-dried soil (<0.15 mm) were weighed onto Whatman QM-A quartz filter material in the cup of a 5.5-cm-diam. polypropylene Büchner funnel and leached under vacuum with two 25-mL aliquots of 0.01 M CaCl2 to remove mineral N. Soil moisture content was adjusted to 60% of water-holding capacity [determined as described by Bremner and Shaw (1958)], and the cup containing the moistened soil sample was then placed in an incubator maintained at 25°C and 85 to 90% relative humidity. After 0, 1, 2, 4, 8, or 12 wk, quadruplicate samples were transferred to 125-mL polyethylene bottles, and mineral N was extracted by shaking the soil sample with 50 mL of 2 M KCl for 1 h on a reciprocal shaker and filtering the resulting suspension through Whatman no. 42 filter paper under vacuum. The bottle was rinsed twice with approximately 10 mL of deionized water from a wash bottle, so as to ensure complete recovery of the incubated soil sample. The soil was transferred with the filter paper to a 125-mL Erlenmeyer flask fitted with a 24/40 standard-taper joint, and hydrolysis was performed as described previously. At biweekly intervals, samples incubated for 4, 8, or 12 wk were leached under vacuum with two 25-mL aliquots of 0.01 M CaCl2, and all leachate from a single sample was combined. Following leaching, the soil moisture content was readjusted to 60% of water-holding capacity, and the samples were returned to the incubator. To evaluate the effects of leaching on the production of mineral N and distribution of hydrolyzable soil N, an additional set of samples was incubated for 12 wk, without biweekly leaching. All samples were weighed periodically during incubation to check soil moisture content, which was found to remain constant under the conditions that existed inside the incubator. Extracts and leachates were analyzed for NH4–N and (NO3 + NO2)-N by accelerated diffusion methods (Khan et al., 1997).
Statistical Analysis
Data from replicate determinations or incubations were characterized by computing means and standard deviations, and mean values were compared on the basis of a least significant difference (LSD) at the 0.001 probability level. Simple correlation analyses were performed to quantify the relationship of different soil N fractions, and also organic C, to check-plot yield or the percentage response of corn to N fertilization.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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45–56 kg ha-1) and exchangeable K (290–335 kg ha-1). Two of the 11 nonresponsive soils were somewhat low in pH, and this was also the case for exchangeable K. Yet these occurrences are probably of no signficance, because some of the responsive soils were also somewhat low in pH, P, or K, including the two that were most responsive to N fertilization. In several studies in the midwestern and northeastern USA, manuring has been identified as a major factor when corn does not respond to N fertilization (e.g., Fox et al., 1989; Roth and Fox, 1990; Meisinger et al., 1992; Schmitt and Randall, 1994). The same trend was reported by Brown et al. (1993), but in this case, a lack of N response was also observed for several sites that received no manure for the growing season(s) studied or within the preceding 3 yr. Soils of both types were included among the 11 nonresponsive samples studied in our work (Table 1). Of this group, seven had been manured, but the rate of manure N application varied from 85 to more than 2500 kg ha-1 and far exceeded crop N requirements at three sites that had been used for manure disposal (Soils 1, 2, and 5). One of the remaining nonresponsive soils was from a site that had been cropped to alfalfa (Medicago sativa L.) prior to corn (Soil 3), and the lack of response can be attributed to mineralization of alfalfa residue (El-Hout and Blackmer, 1990; Bundy and Andraski, 1993; Schmitt and Randall, 1994). The elevated levels of available P and/or K observed for the three remaining nonresponsive soils (8, 10, and 11) suggest that manure may have been applied in the past, although all of the responsive soils were also excessive in one or both of these nutrients.
Soil testing for NO3, either before (PPNT) or after (PSNT) planting, is currently considered the best option for identifying sites where N fertilization will be ineffective in producing a yield response (Bundy and Meisinger, 1994). Both the PPNT and the PSNT were performed by Brown et al. (1993), and the data thereby obtained for the 18 soils studied in our work are included in Table 1. Assuming a critical value of 16 mg NO3–N kg-1 for the PPNT (Schmitt and Randall, 1994) and 21 mg NO3–N kg-1 for the PSNT (Fox et al., 1989; Bundy and Andraski, 1993), both tests correctly identified all seven of the responding soils but failed with the majority of nonresponsive soils. Examination of the data in Table 1 reveals that, of the latter group, a lack of response was detected in three of 11 cases by the PPNT, and in five of 11 cases by the PSNT. Interestingly, neither test was effective with the majority of manured sites, including one used to dispose of liquid swine (Sus scrofa domesticus) manure (Soil 2). The presence of available N from alfalfa (Soil 3) was detected by the PSNT, but not by the PPNT.
The fact that the PSNT was more effective than the PPNT in identifying sites where there was no response to N fertilization can be attributed to the 2-mo delay in soil sampling, which improves the likelihood of detecting mineralization that occurs early in the growing season (Magdoff, 1991). Nevertheless, more than 50% of the nonresponsive sites were undetected by the PSNT, and no improvement was achieved by including exchangeable NH4–N, as recommended for manured sites by Meisinger et al. (1992). The frequent failure of the PPNT and PSNT is no doubt due to the transient nature of mineral N in soils. Nitrate concentrations, for example, depend on numerous N-cycle processes, including mineralization, immobilization, nitrification, denitrification, leaching, and plant uptake. As a result, soil NO3 levels tend to be highly dynamic in a humid region, and therefore a one-time test such as the PPNT or PSNT is apt to be of little value for predicting crop N availability throughout the growing season.
Ideally, a soil test for N would estimate a labile organic fraction that supplies the plant through mineralization. This approach would have the major advantage over NO3 testing that soil test levels would depend on fewer N-cycle processes; therefore, these levels would be less prone to temporal and spatial variability, so that N availability could potentially be predicted on the basis of a one-time test, regardless of soil type or management. Moreover, the time of soil sampling would be much less critical than with NO3 tests, and samples could be stored for later analysis.
Numerous chemical methods have been proposed to estimate the availability of soil organic N (Bremner, 1965; Keeney, 1982; Stanford, 1982; Bundy and Meisinger, 1994), but these have been based on an empirical approach, and their use has been very limited due to low correlations with crop N uptake and/or the production of mineral N during soil incubations. A more rational approach would require chemical fractionation of soil organic N to identify a labile pool; however, little progress has been made in this respect, largely because of fundamental defects in the available methodology that vitiated analyses for amino sugar N and amino acid N. These defects were identified and eliminated through a substantial effort that ultimately led to simple diffusion methods of fractionating the N in soil hydrolysates (Mulvaney and Khan, 2001), which were utilized in the present project.
Given the wide variety of soil types and management practices represented by the 18 samples studied in our work, there was good reason to expect a difference in their content and distribution of hydrolyzable N. This is exactly what was observed, as the data (Table 2) show a fivefold range in total hydrolyzable N, a 13-fold range in amino acid N, a threefold range in hydrolyzable NH4–N, and an 11-fold range in amino sugar N. In each case, the highest value was obtained with the most heavily manured soil that also had the highest content of organic C and total N (soil 5), and the lowest value was obtained with an unmanured soil that had the lowest content of organic C and total N (soil 18). Amino acid N usually exceeded NH4–N or amino sugar N, and was correlated more highly with organic C (r = 0.92***), total N (r = 0.93***), or total hydrolyzable N (r = 0.93***), as compared with amino sugar N (r = 0.65* to 0.74**). This suggests that amino acids and amino sugars may differ in the extent to which they occur as stable humic forms, and hence in their tendency to undergo mineralization.
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The clear distinction between responsive and nonresponsive soils in their concentrations of amino sugar N is truly remarkable, given the wide range in soil properties and management, and the fact that these samples had been stored for 7 to 9 yr prior to hydrolysis. Further evidence of a close relationship between soil amino sugar content and nonresponsiveness to N fertilization is provided by Table 3, which shows the r-values obtained when simple correlations were performed to relate the check-plot yields and fertilizer-response data in Table 1 to different forms of hydrolyzable soil N (Table 2). Among the parameters tested, the highest statistical significance (P < 0.001) was obtained for amino sugar N. The correlation was positive for check-plot yield and negative for N-fertilizer response, as would be expected for any potentially available form of soil N. At least one significant correlation was also obtained for organic C, total hydrolyzable N, and amino acid N, despite the fact that responsive and nonresponsive soils were not resolved on the basis of these parameters.
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Among nonmanured soils that did not respond to N fertilization, the concentration of amino sugar N was lower for a soil previously cropped to alfalfa (Soil 3) than for three soils under continuous corn or in a corn–soybean [Glycine max (L.) Merr.] rotation (Soils 8, 10, and 11). None of these soils had been manured for at least 3 yr prior to the growing season studied, but two of them (Soils 8 and 10) were collected in the vicinity of an operational or former dairy farm, and their elevated content of available P and/or K (Table 1) suggests that manure had been applied sometime in the past. The accumulation of amino sugar N in Soils 3 and 11 is probably related to N carryover from the previous crop. Such accumulation can almost certainly be attributed to microbial decomposition of the residue, because amino sugars occur extensively in cell wall material produced by bacteria, actinomycetes, or fungi (Parsons, 1981), whereas these compounds are very minor constituents of higher plants (Sharon, 1974). The resulting buildup of labile N would promote mineralization, and thereby lead to soil NO3 accumulation and a lack of fertilizer-N responsiveness, as is often observed when corn follows alfalfa (El-Hout and Blackmer, 1990; Bundy and Andraski, 1993; Schmitt and Randall, 1994). In the case of Soil 11, the elevated concentration of amino sugar N may well be related to the fact that corn followed a no-till soybean crop, as recent work by Guggenberger et al. (1999) indicates that microbial growth is favored by no-till systems, which leads to an increase in the occurrence of glucosamine, muramic acid, and other amino sugars.
The seven responsive soils varied considerably in their response to N fertilization, and there is some indication that the variation was related to their content of amino sugar N, suggesting the possibility of a quantitative soil test as well as a means of detecting nonresponsive sites. Within this group, the most responsive soil (Soil 18) had the lowest concentration of amino sugar N, whereas two of the three least responsive soils (Soils 13 and 16) had the highest concentrations of amino sugar N. The remaining soil of the latter group (Soil 12) had been subject to drought while cropped to no-till soybean in the year prior to the growing season studied, and the reduced amount of residues may account in part for the greater N response and much lower concentration of amino sugar N, as compared with a nonresponsive soil with the same cropping history (Soil 11).
If amino sugars are more labile than other fractions of hydrolyzable soil N and supply the bulk of plant-available N, then the production of mineral N should be greater for nonresponsive soils having a high concentration of amino sugar N than for responsive soils having a low concentration of amino sugar N, when incubation is performed under conditions that promote mineralization. This is exactly what was observed in a 3-mo laboratory incubation experiment using three nonresponsive soils (Soils 3, 5, and 6) and two responsive soils (Soils 16 and 18), which were leached initially and at biweekly intervals to reduce immobilization. Table 4 shows that, regardless of the incubation interval, recoveries of mineral N were significantly greater (P < 0.001) for each of the nonresponsive soils than for either responsive soil. Moreover, production of mineral N by the five soils always decreased in the order: Soil 5 > Soil 6 > Soil 3 > Soil 16 > Soil 18. Examination of Table 2 reveals that these soils followed almost the same order in their concentrations of amino sugar N.
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Critical examination of Table 4 reveals that, with each of the soils incubated, the concentration of mineral N did not change at a uniform rate after rewetting, but in stages. Changes during the first week were of limited magnitude, and reflected either net mineralization or immobilization. A progressive increase in mineral N concentration was observed after incubation for 2 wk, 1 mo, or 2 mo, and the increase was considerably larger for nonresponsive than for responsive soils. With four of the five soils, a marked decline occurred in the recovery of mineral N when the period of incubation was increased from 2 to 3 mo, indicating net immobilization. No such decline was observed with Soil 5, which had received the highest application of manure.
To explore the relationship between different forms of hydrolyzable soil N and the production of mineral N under laboratory conditions, N-distribution analyses were performed in conjunction with the aerobic incubations that generated the data in Table 4. The concentrations of hydrolyzable N are plotted in Fig. 1, which shows that the various forms followed different trends during incubation. Interestingly, amino acid N tended to be less variable than total hydrolyzable N, NH4–N, or amino sugar N, and regardless of the incubation period, there was no apparent relationship between recoveries of amino acid N and mineral N. The latter disparity is particularly evident for a responsive soil (Soil 16) and two nonresponsive soils (Soils 3 and 6), which were in close agreement throughout the entire study in their concentrations of amino acid N, but differed markedly in mineral N concentration (Table 4).
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Changes in hydrolyzable N were more modest for responsive than for nonresponsive soils, which can no doubt be attributed to a limited supply of substrate for microbial growth and activity. This limitation would have been particularly serious for Soil 18, owing to a low content of organic C and N, and probably accounts for the fact that, when this soil was incubated, a clearly defined, inverse relationship existed between concentrations of amino sugar N and mineral N. No such relationship was found for any other form of hydrolyzable soil N.
If amino sugars serve as the major source of readily mineralizable soil N, then prolonged incubation should have led to a net decrease in amino sugar N, provided that immobilization was controlled by periodic leaching. Table 5 shows that this trend was indeed observed with all five of the soils studied, and that a net decrease also occurred in total hydrolyzable N and NH4–N, but not necessarily in amino acid N. The latter finding is consistent with previous evidence that amino acid N is less mineralizable than amino sugar N.
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In summary, the work reported implicates the soil amino sugar fraction as a key factor affecting the responsiveness of corn to N fertilization. This conclusion is based on (i) a clear distinction in N distribution analyses that was observed between soils from seven responsive and 11 nonresponsive sites in a N-response study; (ii) very highly significant correlations between amino sugar N and check-plot yield or fertilizer-N response; and (iii) markedly greater production of mineral N by nonresponsive than responsive soils during laboratory incubations, which was always accompanied by a net decrease in amino sugar N. On the basis of amino sugar N, Fig. 2 shows that all 18 soils were classified correctly as responsive (<200 mg kg-1) or nonresponsive (>250 mg kg-1) to N fertilization. If a routine soil test can be developed to estimate amino sugar N, the means should exist to identify sites where corn can be grown profitably without the use of N fertilizer.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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*, **, *** Significant at the 0.05, 0.01, and 0.001 probability levels, respectively.
Received for publication October 27, 2000.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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